1. Field of the Invention
The present invention relates to an image processing apparatus and a method for controlling the same. More particularly, the invention relates to an image processing apparatus having a function of identifying whether an input image is a color image or a monochromatic image, and a method for controlling the image processing apparatus. The invention also relates to an image forming apparatus using the image processing apparatus.
2. Description of the Related Art
In a color image forming apparatus, particularly a color copier, or a color copying apparatus realized by combining a color scanner, a computer and a color printer, or the like, when forming and outputting an image of a monochromatic original, copying is sometimes performed using four colors, i.e., C (cyan), M (magenta), Y (yellow) and K (black). However, in the case of a color copier or a color copying apparatus using a color laser-beam printer, it is desirable to perform copying with a black monochrome for a monochromatic original, in order to increase the life of a drum and reduce the amount of consumption of a toner. The situation is the same in the case of a color copier or a color copying apparatus using a color ink-jet printer.
Accordingly, it is desired that a color-image forming apparatus has a function of identifying whether an original is a color original or a monochromatic original. Conventionally, determination of the type of an original is mainly performed according to simple evaluation such that color pixels (pixels determined to be chromatic) of an input original are added, and the added value is statistically evaluated or compared with at threshold.
However, when determining a color pixel based on color components (for example, R, G and B luminance signals) of each pixel of an image of an original read by an original-input device, if a reading element for reading a color component at the same position cannot perform a reading operation at completely the same pixel position, i.e., if reading is performed by being shifted by a very small distance (hereinafter this state is termed “color deviation”), an edge portion of a black line is detected to be chromatic.
Particularly, since, recently, accuracy of reading (resolution) of an image scanner used as an image input device is increasing, the above-described problem often attracts notice.
Even if accuracy of a reading position is sufficient, a color component (a pseudo-color) such as one generated when accuracy in a reading position is insufficient is generated at a position near an edge of a black line, depending on variations in MTF (modulation transfer function) characteristics of a lens at different wavelengths. The fact that the generation of a pseudo-color component due to color deviation or variations in MTF characteristics of a lens will cause no problem when reading an original having a uniform color (for example, white paper) will be understood from the above-described reason.
The problem is that an achromatic pixel is determined to be chromatic due to a pseudo-color generated due to color deviation or variations in MTF characteristics of a lens at a portion near an edge of a black character or a black fine line.
In the above-described conventional processing, however, since the same threshold is used for determining a degree of continuity in a main scanning direction and for determining a group of consecutive color pixels in counting in the main scanning direction and counting in a sub-scanning direction, accuracy of determination cannot be changed between the main scanning direction and the sub-scanning direction. As a result, sometimes, optimum determination reflecting the characteristics of an image scanner and differences among image scanners cannot be performed.